Crucially, the research led by the University of York and reported in *NatureMaterials*, shows that oxidation of metals - the process that describes,for example, how iron reacts with oxygen, in the presence of water, to formrust - proceeds much more rapidly in nanoparticles than at the macroscopicscale. This is due to the large amount of strain introduced in thenanoparticles due to their size which is over a thousand times smaller thanthe width of a human hair.

Improving the understanding of metallic nanoparticles - particularly thoseof iron and silver - is of key importance to scientists because of theirmany potential applications. For example, iron and iron oxide nanoparticlesare considered important in fields ranging from clean fuel technologies,high density data storage and catalysis, to water treatment, soilremediation, targeted drug delivery and cancer therapy.

The research team, which also included scientists from the University ofLeicester, the National Institute for Materials Science, Japan and theUniversity of Illinois at Urbana-Champaign, USA, used the unprecedentedresolution attainable with aberration-corrected scanning transmissionelectron microscopy to study the oxidisation of cuboid iron nanoparticlesand performed strain analysis at the atomic level.

Lead investigator Dr Roland Kröger, from the University of York'sDepartment of Physics, said: "Using an approach developed at York andLeicester for producing and analysing very well-defined nanoparticles, wewere able to study the reaction of metallic nanoparticles with theenvironment at the atomic level and to obtain information on strainassociated with the oxide shell on an iron core.

"We found that the oxide film grows much faster on a nanoparticle than on abulk single crystal of iron - in fact many orders of magnitude quicker.Analysis showed there was an astonishing amount of strain and bending innanoparticles which would lead to defects in bulk material."

The scientists used a method known as Z-contrast imaging to examine theoxide layer that forms around a nanoparticle after exposure to theatmosphere, and found that within two years the particles were completelyoxidised.

Corresponding author Dr Andrew Pratt, from York's Department of Physics andJapan's National Institute for Materials Science, said: "Oxidation candrastically alter a nanomaterial's properties - for better or worse - andso understanding this process at the nanoscale is of critical importance.This work will therefore help those seeking to use metallic nanoparticlesin environmental and technological applications as it provides a deeperinsight into the changes that may occur over their desired functionallifetime."

The experimental work was carried out at the York JEOL Nanocentre and theDepartment of Physics at the University of York, the Department of Physicsand Astronomy at the University of Leicester and the Frederick-SeitzInstitute for Materials Research at the University of Illinois atUrbana-Champaign.

The scientists obtained images over a period of two years. After this time,the iron nanoparticles, which were originally cube-shaped, had becomealmost spherical and were completely oxidised.

Professor Chris Binns, from the University of Leicester, said: "For manyyears at Leicester we have been developing synthesis techniques to producevery well-defined nanoparticles and it is great to combine this technologywith the excellent facilities and expertise at York to do such penetratingscience. This work is just the beginning and we intend to capitalise on ourcomplementary abilities to initiate a wider collaborative programme."

The research was supported by a Max-Kade Foundation Visiting Professorshipstipend to Dr Kröger and financial support from the World UniversitiesNetwork (WUN). The Engineering and Physical Sciences Research Council(EPSRC) funded the initial stages of the project (EP/D034604/1).

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About University of YorkThe University of York was founded in 1963 with 200 students. Since then, it has expanded to 10,000 students and has over 30 academic departments and research centres.

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From its inception, the University has concentrated on strong viable departments and teaching and research of the highest quality. The quality of York's teaching has received many accolades. York and Cambridge top the teaching league with the highest scores in official teaching assessments.